H3C Technologies H3C SecPath F1000-E - Manual (Page 16)

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When performing an RPF check, a router searches its unicast routing table, MBGP routing table, and
multicast static routing table at the same time. The specific process is as follows:
1. The router first chooses an optimal route from each of the unicast routing table, MBGP routing table,
and multicast static routing table:
• The router automatically chooses an optimal unicast route by searching its unicast routing table,
using the IP address of the “packet source” as the destination address. The outgoing interface in the
corresponding routing entry is the RPF interface and the next hop is the RPF neighbor. The router
considers the path along which the packet from the RPF neighbor arrived on the RPF interface he
shortest path that leads back to the source.
• The router automatically chooses an optimal MBGP route by searching its MBGP routing table,
using the IP address of the “packet source” as the destination address. The outgoing interface in the
corresponding routing entry is the RPF interface and the next hop is the RPF neighbor.
• The router automatically chooses an optimal multicast static route by searching its multicast static
routing table, using the IP address of the “packet source” as the destination address. The
corresponding routing entry explicitly defines the RPF interface and the RPF neighbor.
2. Then, the router selects one from these three optimal routes as the RPF route. The selection process
is as follows:
• If configured to use the longest match principle, the router selects the longest match route from the
three; if these three routes have the same mask, the router selects the route with the highest priority;
if the three routes have the same priority, the router selects a route as the RPF route according to the
sequence of multicast static route, MBGP route, and unicast route.
• If not configured to use the longest match principle, the router selects the route with the highest
priority; if the three routes have the same priority, the router selects a route as the RPF route
according to the sequence of multicast static route, MBGP route, and unicast route.
NOTE:
The above-mentioned “packet source” means different things in different situations:
• For a packet traveling along the shortest path tree (SPT) from the multicast source to the receivers or the
rendezvous point (RP), the “packet source” for RPF check is the multicast source.
• For a packet traveling along the rendezvous point tree (RPT) from the RP to the receivers, the “packet
source” for RPF check is the RP.
• For a bootstrap message from the bootstrap router (BSR), the “packet source” for RPF check is the BSR.
For more information about the concepts of SPT, RPT and BSR, see PIM Configuration in the IP Multicast
Volume.
Implementation of RPF check in multicast
Implementing an RPF check on each received multicast data packet would bring a big burden to the
router. The use of a multicast forwarding table is the solution to this issue. When creating a multicast
routing entry and a multicast forwarding entry for a multicast packet, the router sets the RPF interface of
the packet as the incoming interface of the (S, G) entry. Upon receiving an (S, G) multicast packet, the
router first searches its multicast forwarding table:
1. If the corresponding (S, G) entry does not exist in the multicast forwarding table, the packet is
subject to an RPF check. The router creates a multicast routing entry based on the relevant routing
information and adds the entry into the multicast forwarding table, with the RPF interface as the
incoming interface.
• If the interface on which the packet actually arrived is the RPF interface, the RPF check succeeds and
the router forwards the packet to all the outgoing interfaces.